The present invention relates to a tracking following apparatus for an optical disk device, and in particular, to a tracking following apparatus for an optical disk device having two systems of actuators including a fine actuator and a coarse actuator.
In an optical disk device, a laser beam is focused through an objective into a light spot to read and to write data from and on an optical disk. In a device of this type, when an optical disk installed therein is rotated, the disk is attended with disk eccentricity of at most about 50 micrometers. The disk eccentricity leads, during the operation of the disk device, to displacement of tracks according to a period primarily related to the disk rotation frequency. Since the track pitch is about one micrometer, the tracking precision of the light spot is required to be about 0.1 micrometer or less.
To achieve such a high tracking precision for the light spot, there has been conventionally employed an optical disk device having a fine actuator which controls the objective such that the light spot continuously conducts a fine tracking operation with respect to the tracks of the optical disk and a coarse actuator which compensates for an error appearing in an interval of the rotation cycle of the optical disk.
For detecting a relative positional error between the light spot and a track on the optical disk, a light sensor senses light of the light spot reflected from the disk surface to attain a track error signal representing the relative positional error.
When displacement of the objective disposed on the fine actuator from a reference position of the objective becomes about several tens of micrometers (.mu.m) or more, there appears in the track error signal a noise which cannot be ignored, and hence the relative positional error between the light spot and the pertinent track cannot be exactly sensed.
To overcome this difficulty, the displacement of the objective is required to be equal to or less than about several tens of micrometers.
To achieve the tracking following control for the light spot, there is ordinarily used a circuit having functions, for example, a function to compensate for lead of phase according to a tracking error signal so as to produce signals to drive the fine and coarse actuators, thereby driving these actuators.
With an increased gain to drive the fine actuator, the displacement of the objective becomes greater at the disk rotary frequency. As a result, the objective displacement exceeds about several tens of micrometers (.mu.m) and a noise which cannot be ignored appears in the track error signal. Consequently, it is impossible to achieve a correct tracking following operation.
On the other hand, increasing the gain to drive the coarse actuator, there occurs unexpected vibration of the coarse actuator not considered in the system design stage and hence the light spot is displaced from the track. For these reasons, the gain of the coarse actuator is limited.
Consequently, there has been proposed, in place of the device increase the gain of the fine or coarse actuator, a tracking control system to conduct a feedforward compensation. For example, there has been described in the JP-A-3-127337 a tracking control system in which a waveform signal of the head vibration due to eccentricity of the optical disk is beforehand memorized when the control operation is started so as to carry out the feedforward compensation by sending the vibration waveform signal to the coarse actuator system.
Moreover, for example, in the JP-B2-60-57085, the JP-A-1-130369, and pages 498 to 504 of the Journal of Institute of Television Engineers of Japan, Vol. 43, No. 5 (1989), there has been described a tracking control system conducting the feedforward compensation in which a tracking error signal related to a rotation of the optical disk is beforehand memorized such that the stored error signal is added to an error signal obtained in the subsequent rotation of the disk, thereby using the resultant signal as a tracking control signal. Until the sensed error signal becomes zero, the error signal of each subsequent disk rotation is accumulated to achieve a so-called repetitive control system.
However, in such a tracking control system using the repetitive control system as the feedforward compensation, to attain a value of feedforward compensation in which the error signals are accumulated for many disk rotations for a high-precision tracking, there are necessitated a multiplicity of learning operations consuming a long period time.